Added formattable concept

[matt77hias]

No description provided.

[vitaut]

Thanks for the PR. It looks good but please address the inline comment.

[vitaut]

Could you also add a test case to base-test?

[matt77hias]

I used static_asserts without a message (C++17) for the concept (C++20) tests.
Is there a reason why is_formattable does not have some checks for the usual suspects that are formattable?

I am not familiar with the CI test setup, but do we already have C++20 coverage?

[phprus]

@vitaut, Maybe makes sense to reintroduced tests for g++-11 and C++20 into CI?
CI currently does not have tests for g++ and C++20.

It was removed in this commit:

0b5287f#diff-21364b2e6fae1f2875cee1ab3daefb0685403687eaf8bc32b5c6eacda351c9d3

[vitaut]

Is there a reason why is_formattable does not have some checks for the usual suspects that are formattable?

I don't think the tests need to be exhaustive. It's enough to cover important categories of formattable types. We don't need to test every type that has a user-defined formatter.

I am not familiar with the CI test setup, but do we already have C++20 coverage?

We do.

Maybe makes sense to reintroduced tests for g++-11 and C++20 into CI?

I removed g++-11 because its installation was broken but a PR to reintroduce it (or another version that supports C++20) would be welcome.

[matt77hias]

I don't think the tests need to be exhaustive. It's enough to cover important categories of formattable types. We don't need to test every type that has a user-defined formatter.

I agree, but I would have expected some static_assert(is_formattable<...) alongside all these static_assert(!is_formattable<...).